M. Sc. Dissertation

M. Sc. Dissertation Study on the eruption process in Arenal Volcano, Costa Rica based on the ground deformation in1982-1998. Enrique HERNANDEZ Department of Earth & Environmental Sciences Graduate School of Environmental Studies Nagoya University September 2010 ABSTRACT Arenal volcano is located in the northwest-trending Costa Rican volcanic arc and it has been active in the last 42 years. Eight ground-tilt stations were established around the perimeter of the volcano as a project of Observatory Volcanology and Seismology from Costa Rica, Universidad Nacional (OVSICORI-UNA) since 1982. We employed single setup leveling (SSL), EDM (five lines) and two continuous GPS sites for measuring ground deformation on Arenal. Large tilt vectors such as 62.2 µradian/year were observed in the west flank of the summit. We assumed a spherical pressure source by Mogi solution to explain the observed crustal deformation in 1982-1998; we divide the 16-year into three periods of 1982-1992, 1993 (August 28, 1993 pyroclastic flows), and 1994-1998. Four source parameters were estimated using a grid-search scheme. According to our results, a spherical source is located 1km west of the summit with a depth of 1.2km, and the deflated volume change rates of −0.57 ×106 m3/year for the first period, −1.00 ×106 m3/year for the second period, and −0.55 ×106 m3/year for the last period. Since 1982, the SSL data has been revealing a continued deflation in the pressure source for 16 years beneath the summit. Detected by EDM and GPS, line lengths are consistent with our model. In the previous studies, spherical pressure sources were proposed based on geodetic data they suggested shallow magma source between 2.1km to 1.8km beneath the summit and deflation volume change between−5.6 ×106 m3/year (June to December 1996) to −0.2 ×106 m3/year (1995-2000). Comparing with previous studies, we have some conclusions on the eruption process in Arenal volcano; the volcano continues the laver and pyroclastic flows for the last 42 years. However, deflation is going at the shallow pressure source since 1976. The deflation volumes are estimated between −0.2 ×106 m3/year and −1.0 ×106 m3/year, except the short period observation of only six months. There is a possibility that another pressure source exists in the depth, because of SSL accuracy is ±5µradians, we cannot detect tilt made by a source deeper than 3.5km. 0 INTRODUCTION Arenal Volcano Tectonic and volcanological setting in Arenal Volcano Arenal is small strato volcano, with a volume of only 15km3 and it is located in Costa Rica along the Central American volcanic arc. The activity started around 7000 year ago and is still in the process of building its 1,670m cone by alternating periods of major explosive eruptions with lava flows that stabilize the loose material on its cone. Arenal began the longest current ongoing eruption of any volcano worldwide in 1968 with a violent Plinian blast and resultant eruptions columns, followed by small pyroclastic flows and rapid effusion of lavas (Reagan et al., 1987). "Cerro" Arenal as local residents called it began explosive activity that literally blew the west side off the volcano. Two villages at the foot of the volcano-- Pueblo Nuevo and Tabacón were completely destroyed and 78 people were killed. Between July 29-31, 1968, three new craters were formed on the western flank of the volcano and a fifteen-square kilometer area was devastated. Arenal lies in 30km NW of the Quesada Sharp Contortion which is a tear fault in sub ducting Cocos Plate that separates steep slab dips of 60° to ~100km beneath the volcanoes in Guanacaste from shallower dips angle of 40°, 80km beneath the volcanoes in the central Valley (Guendel and Protti, 1998) (Figure 1). Arenal volcano is a part of the Arenal-Chato volcanic system, which lies between the Cordillera de Guanacaste to the northwest and the Cordillera Central to the southeast. The volcanic system is on a north-east-trending system of normal faults that form a boundary between the Arenal Graben and the Cordillera de Tilaran, a Tertiary volcanic range (Borgia et al., 1988). The oldest rocks exposed are Miocene continental shelf deposits of the Venado Formation overlain by Miocene-Pliocene rocks of the Aguacate Volcanic Group (Borgia et al., 1988) (Figure 2). Deposited on top of the Aguacate Volcanic Group are Quaternary volcano-sedimentary deposits and local Holocene alluvium. The Holocene volcanic system includes the Arenal volcanic edifice to the northwest and the Chato volcanic edifice to the southeast (Borgia et al., 1988). The volcanic activity within the system has migrated to the northwest from Cerro Chato, which last erupted about 3550 BP to Arenal, which began erupting about 2900 BP (Borgia et al., 1988). The stratigraphic 1 record suggests two dominant types of volcanic activity: Plinian-Sub-Plinian eruptions and Peléan eruptions followed by extensive lava effusion (Borgia et al., 1988). The area around Arenal Volcano is a high-risk zone--this volcano has been active for the past 40 years with regular eruptions providing tourists excellent displays of nature's awesome power. The volcano has become an important tourist attraction for Costa Rica. Several hotels, restaurants and other tourist-oriented ventures have sprung up very close to Arenal in recent years. Previous Studies of Arenal’s Ground Deformation In previous studies, several different models were applied to discuss pressure source under Arenal Volcano. Wadge (1983) analyzed data from four SSL stations located in the western flank of the volcano for two years from 1976 to 1978. All four stations showed downward tilting towards the summit of the volcano, with the magnitude of tilt increasing with proximity to the summit (Wadge, 1983). Using a finite element model to infer the depth of Arenal’s magma source, He obtained a shallow magma source of less than 2km below the summit; the volume of extruded material, however, did not match up the modeled magma chamber size. Finally, he proposed that the loading caused by the extruded lava depressed the west flank of the volcano causing the observed tilting (Wadge, 1983). Since 1995, University of California at Santa Cruz (UCSC) and OVSICORI-UNA established a network consisting with four tiltmeters and two continuous GPS stations on the north and south flanks of the Arenal Volcano (Figure 3). Under the same experiment, Hagerty et al., (1997) analyzed seismic, acoustic, and GPS data on Arenal volcano from 1995 to 1997. He suggests that the data shows shortening across a north-south baseline consistent with deflation of a shallow, non-replenishing magma chamber. They did a preliminary analysis of the seismic and acoustic data, which suggested that the source of tremor and eruptions might be magmatic degassing with bubbles periodically coalescing into large gas slugs that rise and explode beneath the summit. Schapiro (2000) inverted for the best fitting source model from the tilt and GPS data in the period of June-December 1996 using a Monte Carlo optimization technique. He concluded that a dike and a spherical source model best fits his observed tilt and GPS data 2 with a Mogi source depth of 2.01km and a decrease in volume of−5.6 ×106 m3/year, and a vertical stock with 2.18km long, 2.51km wide, 1.72km depth and with an opening rate of 2.1m/year. Baugh (2007) analyzed tilt and GPS data collected on Arenal from 1995 to 2000. She shows results with a magma chamber located northwest of the summit at a depth of 1.8km with a volume change of−0.2 ×106 m3/year. She concluded that tilt data have a varied through time with radial tilt toward the volcano (deflation) at all the sites observed between 1995-1999 changing to a circular pattern of inflation, with a radius of approximately 2500m from the summit, surrounded by deflation elsewhere. Arenal Single Setup leveling network description Eight ground-tilt stations were established on the flanks of the Arenal Volcano in 1982 as a project of the OVSICORI-UNA (Figure 3). We employed the “dry tilt” technique for measuring ground deformation on Arenal. This technique, developed at the Hawaii Volcano Observatory in the late 1960s, utilizes precise leveling methods to determinate changes in elevation of benchmarks affixed firmly in the ground (Yamashita, 1981). It is also called “single setup leveling” (SSL) because it is a special type of tilt leveling in which relative displacements within a small array of marks are measured from a single instrument setup to determinate local ground tilt. In our network, a dry tilt station consists of three benchmarks arranged in a roughly equilateral triangle with sides of about 40m. The level is set up in the centre with the distance of 23.1m from each vertex and the staff placed on each of the three benchmarks in turn, and their relative altitudes are measured with a high precision. Next time the station is measured, any tilt of the ground in the intervening period will show up as a slight difference in relative altitudes of the three benchmarks. A complete description of the original spirit-level tilt technique can be found in Kinoshita and others (1974), or Yamashita (1981) also presented a detailed description. The ground-tilt stations established by OVSICORI-UNA in the area between 350m and 750m of altitude with a maximum radio around 5km from the summit of Arenal are described on the Table 1. In our case, the numbers of stations were determined by practical 3 considerations such as accessibility and the time required surveying the network relative to the desired survey frequency.

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